Tomatoes need a long, warm season, and in most of the country the outdoor season is simply not long enough if you sow direct. Starting indoors is the standard way to grow them: it gives you sturdy, stocky plants that are already flowering by the time the weather turns, instead of small seedlings still finding their feet in June.
Starting tomatoes indoors is the standard approach for this crop, and this guide follows it step by step. Germination takes 7–14 days, maturity 60–85 days, spacing is 24–36 inches and soil pH 6.0–6.8.
Tomatoes at a glance
| Growing factor | What tomatoes need |
|---|---|
| Days to germination | 7–14 |
| Sowing method | start indoors |
| Days to maturity | 60–85 |
| Soil pH | 6.0–6.8 |
| Spacing | 24–36 inches apart |
| Sun | full sun |
| Cold tolerance | tender |
When to start tomato seeds indoors
Sow 6–8 weeks before your last frost date. In Zones 3–5 that usually means late winter, in Zones 6–8 early spring, and in Zones 9–10 late winter to very early spring. Zones 11–13 are frost-free, so you can start seed through much of the year — aim for the cooler months, when young plants are not fighting extreme heat, and check your local extension service for the best local sowing window.
Work backwards from your own last frost date: find it with our frost-date calculator, then count back the weeks above. If you are unsure, check your local extension service, and use the seed-starting calculator to work out your sowing date.
What you need
You do not need expensive kit: clean trays, cells or small pots, a bag of seed-starting mix (not garden soil, which is too heavy and can carry disease), seed, labels and a pen, and a watering can with a fine rose or a spray bottle. A heat mat helps if your room is cool, and a clear lid keeps the mix moist until seedlings appear — take it off the moment they do, or damp air invites disease. Our grow-light calculator checks your light setup, and the seed-viability checker tells you whether last year's seed is still worth sowing.
Step by step: starting tomato seeds indoors
- Fill your containers. Fill clean trays or pots with damp seed-starting mix, firm it gently and level the surface — it should feel like a wrung-out sponge.
- Sow the seed. Sow tomatoes seed at about twice the seed's own width deep, spacing seeds so seedlings do not crowd each other, and sow a few extra to allow for failures.
- Label everything. Label each tray with the crop and sowing date before you water — unlabeled trays are the commonest seed-starting mistake.
- Water gently. Water from below by standing the tray in a shallow dish, or mist lightly; heavy watering from above washes seed out of place.
- Keep it warm and covered. Cover with a lid or cling film, keep somewhere warm, check daily, and remove the cover the moment seedlings appear.
- Give light immediately. Move the tray to your brightest spot or under grow lights as soon as seedlings emerge. Burying the stem is the tomato trick that matters most: when you pot seedlings on, set each one deeper than it was growing, up to its first true leaves, because the buried stem grows extra roots.
- Pot on when ready. When seedlings have two or three true leaves, pot the strongest ones into individual pots, handling them by a leaf, never by the stem.
Light for seedlings
Light decides whether your tomatoes seedlings are stocky or stretched, and it matters from the day they emerge. Tomatoes need full sun: indoors, that means your brightest spot — a south-facing windowsill — or a grow light hung just a few inches above the plants. A light too far away is almost as bad as no light at all, so turn trays daily so they do not lean. Pale, tall seedlings need more light, not more feed. Our grow-light calculator checks your setup.
Watering seedlings
Water needs for tomatoes are high: seedlings need steady moisture, never sodden and never bone dry. Water from below where possible, so the surface stays drier and disease has less chance to take hold — a constantly wet tray is the main cause of damping off, which fells seedlings at soil level overnight. Water in the morning and let the top of the mix dry slightly between waterings once seedlings have true leaves.
Hardening off
Hardening off is the week-long process of getting indoor seedlings used to outdoor life — skipping it is the fastest way to lose a tray of good plants. Start about a week before planting out: put seedlings in a sheltered, shady spot outside for a few hours on day one, then bring them in at night. Lengthen the time outside and the sun each day, leaving them out overnight only in the last day or two, when nights are mild, and bring them in if frost, wind or heavy rain is forecast. By the end of the week they should stand in full sun and a breeze without wilting.
Transplanting outdoors
Plant out only once all danger of frost has passed and the soil has warmed. Set each plant deep, up to its first true leaves, and put its stake or cage in at planting time rather than later, when you risk damaging roots.
Good neighbours for tomatoes include basil, marigolds, carrots. Set transplants out on a cloudy day or in the evening, water each one in well, and keep the soil moist for the first week while roots establish — a transplant that dries out early rarely recovers its vigour. Mulch once plants are settled, and check spacing with our plant-spacing calculator.
Common problems and fixes
| Problem | Likely cause | Fix |
|---|---|---|
| Damping off — seedlings collapse at soil level | Mix too wet, poor air flow | Sow thinner, water from below, remove covers once seedlings emerge |
| Leggy, pale seedlings | Not enough light | Move lights closer, give more hours of bright light, keep seedlings cool |
| Slow or patchy germination | Mix too cool or too dry | Keep the tray warm and moist, check seed viability |
| Aphids damage | Pest attack on young plants | Inspect leaf undersides weekly, wash off or hand-pick, encourage beneficial insects |
| Tomato hornworm damage | Pest attack on young plants | Inspect plants early, remove affected leaves, keep plants well watered and unstressed |
Damping off
Damping off makes healthy-looking seedlings keel over at soil level overnight. It thrives in wet, still air, so prevention is everything — clean trays, fresh mix, thin sowing, watering from below and good air movement. A damped-off seedling cannot be saved, so remove it at once.
Aphids and Tomato hornworm
aphids, tomato hornworm, whiteflies, cutworms, blight are the pests most often reported on tomatoes. Check young plants weekly, especially leaf undersides, and act at the first sign of damage. A strong jet of water dislodges many small pests, hand-picking deals with larger ones, and healthy, well-watered plants resist attack far better than stressed ones. Our pest-identifier helper can help identify what you are seeing.
Frequently asked questions
How long do tomato seeds take to germinate?
Tomatoes seed takes 7–14 days to germinate in warm, moist mix. Cooler conditions slow it down, and old or badly stored seed may fail — test it first with our seed-viability checker.
How far apart should I space tomatoes?
Space tomatoes 24–36 inches apart at planting-out time. That spacing keeps air moving and gives each plant room to reach full size — crowding is a common cause of small harvests and more disease.
What soil pH do tomatoes need?
Tomatoes grow best at a soil pH of 6.0–6.8. Test your soil before amending it, adjust over a season rather than guessing, and dig in compost to improve the soil whatever its pH.
Can I start tomato seeds on a windowsill without grow lights?
You can, but only on a very bright, south-facing windowsill, and even then seedlings often stretch. A simple shop light hung a few inches above the plants gives far stockier seedlings, and it is the single best upgrade if your first batch came out leggy.